Electroluminescent device using electroluminescence organic materials (also referred to as “organic EL device”) can realize emitted light of each color in the visible light range by selecting an organic material or by adopting an appropriate organic EL device structure and emitting white light. As a result, the development of electroluminescence devices suitable for purposes such as displays or lighting using an organic EL device is progressing.
An organic EL device is formed by stacking a transparent electrode such as ITO (Indium Tin Oxide), an organic electroluminescence layer (referred to below as “organic EL layer”) formed from a hole transport layer, a light emitting layer or electron transport layer and the like, and a reflective electrode, wherein light emitted in the organic EL layer is irradiated by passing through the transparent electrode. The thickness of the organic EL layer is about 100 nm to 200 nm and the light emitting by the light emitting layer spreads in all directions of 4π when expressed as a solid angle. In addition, light emitted straight to/from a glass substrate side and a part of the light reflected at a boundary between the reflective electrode and organic EL layer is emitted to the glass substrate side.
However, light emitted in parallel to a film surface within the organic EL layer heads towards an end surface of the organic EL layer and is not emitted from the transparent electrode side. In the organic EL device, the refractive index of a material which forms the organic EL layer is high (n=1.8˜1.9), and light entering at a specific angle at a boundary surface with a different refractive index is all reflected. For example, all the light which is reflected at a boundary surface between the organic EL layer and transparent electrode, and at a boundary surface between a glass substrate and air is either wave guided through the interior of the organic EL layer or glass substrate and absorbed within the layers or is emitted from the end surface of the glass substrate so that the emitted light cannot be effectively utilized.
Due to the light which is wave guided through the interior of the organic EL layer, the extraction efficiency (ratio of the amount of light emitted to the glass substrate side to the total amount of emitted light in the organic EL layer) of light generated in the organic EL layer is said to be about 20%. Increasing the extraction efficiency of light generated in the organic EL layer is important for reducing power consumption of an electroluminescence device formed using an organic EL device.
For example, a technology for improving the extraction efficiency by arranging a transparent resin layer between a transparent electrode and a glass substrate of an organic EL device, and burying a cone shaped transparent resin with a different refractive index within the transparent resin layer is disclosed in Japanese Laid Open Patent 2011-124103. In addition, a technology for improving the extraction efficiency by forming a metal electrode having a unique surface obtained by forming a concave-convex surface into a curved surface shape into a reflective electrode of a light emitting device is disclosed in Japanese Laid Open Patent 2007-288143.
However, using a unique optical adjustment film on a light emitting side of an organic EL device as is disclosed in Japanese Laid Open Patent 2011-124103 in order to improve the extraction efficiency is the cause of an increase in costs which is not desirable. In addition, forming a metal electrode having a unique concave-convex surface as a reflective electrode of an organic EL device as is disclosed in Japanese Laid Open Patent 2007-288143 leads to complex manufacturing processes which is also not desirable.
Although it is possible to increase luminosity when light is emitted if a current flowing to an organic EL device is increased, this becomes a problem from the viewpoint of energy conservation since power consumption is also increased by this amount. In addition, the larger the value of a current flowing to an organic EL device, the more the organic EL device deteriorates leading to a reduction in the lifetime of the electroluminescence device.